A period 1 element is an element in the first period (row) of the periodic table. The periodic table is arranged in rows to show repeating properties of the elements. When the atomic number increases, the element have different properties. A new row begins when chemical properties repeat. It means that elements in the same group have similar properties. The first period has less elements than any other periods in the periodic table. There are only two elements in the first period: hydrogen and helium. We can explain why there is less elements in the first row in modern theories of atomic structure. This is because in quantum physics, this period fills up the 1sorbital. Period 1 elements follows the duet rule, they only need two electrons to complete their valence shell. These elements can only hold two electrons, both in the 1s orbital. Therefore, period 1 can have only two elements.

Hydrogen is the most abundant of the chemical elements. The abundance of hydrogen is roughly 75%.[7]Stars in the main sequence are mainly composed of hydrogen in its plasma state. However, there are less hydrogen on Earth. Therefore, hydrogen is industrially produced from hydrocarbons (e.g. methane). We use elemental hydrogen locally at the production site. The largest markets almost equally divided between fossil fuel upgrading, such as hydrocracking, and ammonia production, mostly for the fertilizer market. Hydrogen may be produced from water using the process of electrolysis, but this process is significantly more expensive commercially than hydrogen production from natural gas.[8]

Helium is the second lightest element and is the second most abundant in the observable universe.[31] Most helium was formed during the Big Bang, but new helium is being created as a result of the nuclear fusion of hydrogen in stars.[32] On Earth, helium is relatively rare and is created by the natural decay of some radioactive elements[33] because the alpha particles that are emitted consist of helium nuclei. This radiogenic helium is trapped with natural gas in concentrations of up to seven percent by volume,[34] from which it is extracted commercially by a low-temperature separation process called fractional distillation.[35]